1. Field of the Invention
The present invention relates to electric arc furnaces and, more particularly, to a control system and method for controlling electrodes in electric arc furnaces based on direct power factor regulation.
2. Description of the Prior Art
Typically, electric arc and, therefore, power input to an electric arc furnace is regulated by controlling the positioning of the electrodes in the electric arc furnace either manually or automatically. However, to provide consistent operation of the electric arc furnace, automatic regulation of the electrodes is preferred. In particular, the electric arc, or arc length, of the electrode is controlled by controlling the position of the electrode with respect to the level of molten metal contained in the electric arc furnace. Various types of electrode regulating systems are known in the art such as current regulating systems, impedance regulating systems, resistance regulating systems and power regulating systems. Impedance regulating systems are the most widespread and well-established in the industry. In impedance regulating systems, it is common practice to measure the current and voltage of the electrodes at the tertiary or secondary side of the furnace transformer to determine electrode impedance. The electrode impedance as a process input is then compared with a theoretical impedance value to maintain a constant electric arc, or arc length.
A principal disadvantage with impedance regulating systems is that an xe2x80x9coptimumxe2x80x9d power factor and, thus, an optimum working/operating condition of the electric arc furnace is difficult to achieve and maintain in practice. This is primarily due to the inherent complexity and time consuming efforts required for adjusting the theoretical impedance value to the required or xe2x80x9coptimumxe2x80x9d power factor of the electrode. In addition, the theoretical impedance is a constant and does not consider the frequently changing conditions in the electric arc furnace which necessitates continuous adjustments of the theoretical impedance. Optimum working/operating conditions are typically never met in electric arc furnaces that utilize impedance regulating systems. A typical impedance regulating system for electrodes in an electric furnace is disclosed by U.S. Pat. No. 5,255,285 to Aberl et al. (hereinafter xe2x80x9cthe Aberl patentxe2x80x9d).
The Aberl patent generally discloses an impedance based control system for an electric arc furnace that includes an electrode actuator, a controller for controlling the electrode actuator based on electrode impedance, and an impedance signal generator in which the electrode impedance is calculated. The controller is utilized to compare the electrode impedance with a desired impedance value. The impedance regulating system disclosed by the Aberl patent attempts to overcome the inherent deficiencies with impedance regulating systems by providing a feed back control loop or xe2x80x9ccorrectingxe2x80x9d signal to the controller. However, the xe2x80x9ccorrectingxe2x80x9d signal is nothing more than an indirect measure of the electrode impedance with an allowance for the resistance and inductive reactance of an electric lead connected to the furnace transformer of the electric arc furnace, and thus does little to improve the performance of the overall control system. Impedance type electrode regulating systems are generally time consuming and inefficient methods of control for electrodes in electric arc furnaces.
As discussed previously, electrode regulating systems may be based on such criteria as current, impedance and resistance. However, these values arc only indirect measurements providing indirect information on the power input to the individual electrodes. The most important value, or control criteria, requiring scrutiny and generally ignored in the prior art is electrode power factor.
FIG. 1 illustrates how operating or xe2x80x9csecondaryxe2x80x9d current, operating or xe2x80x9csecondaryxe2x80x9d voltage, impedance and power input are related in electric arc furnaces. FIG. 1 is a circle diagram of operating voltage versus operating current and shows power factor values for five power factor set points. During normal operating conditions, the power input should be maximized and the electrode consumption minimized. The arc length should be stable and over-currents should be avoided. The circle diagram of FIG. 1 shows that maximum available power occurs at a power factor of cos xe2x88x9d=0.707. However, maximum power input does not necessarily result in a maximum rate of heating or optimal operation of the furnace when other factors such as electrode consumption or carbon pickup are considered. For this reason, most electric arc furnaces operate at slightly higher power factors ranging between cos xe2x88x9d=0.72 and 0.78.
Attempts have been made in the prior art to incorporate power factor into a control system for controlling electrodes in electric arc furnaces. However, these attempts have centered on utilizing power factor to control the power source for the electric arc furnace or as a secondary or xe2x80x9ccorrectingxe2x80x9d signal in what otherwise are well-known current or impedance based regulating systems. At best, these attempts have only succeeded in utilizing power factor as an indirect or secondary process input and, hence, are not true power factor based regulating systems.
For example, U.S. Pat. No. 3,435,121 to Jackson discloses an arc power responsive control system for consumable electrode furnaces that utilizes power factor as a control criteria or value for controlling a transformer power source of the furnace, rather than as a control criteria for controlling the positioning of the furnace electrodes. In particular, the transformer power source control circuit disclosed by the Jackson patent includes a power factor transducer which receives an arc current signal and an arc voltage signal from the transformer power source. From the power factor transducer, a signal indicative of the phase angle between the arc current signal and arc voltage signal is fed to a power factor level detector that compares the actual power factor with a desired power factor range. If actual power factor falls outside of the desired power factor range, the power factor level detector provides an output signal proportional to the difference directly to the transformer power source. The output signal provided to the transformer power source identifies which transformer tap should be used on the transformer power source. The output signal is not used as a process input to the electrode control circuit disclosed by the Jackson patent that actually controls the positioning of the electrodes in the furnace. The electrode control circuit disclosed by the Jackson patent is defined by a power transducer which receives the arc current and arc voltage signals from the transformer power source, a power averaging circuit, a power comparison circuit, a power reference source, an amplidyne, and an electrode drive motor. The output signal generated by the power factor level detector is not provided as a process input to this control circuit. Hence, the control system disclosed by the Jackson patent is not configured to change the positioning of the electrodes in the furnace based directly on power factor as a process input to the electrode control circuit.
Another prior art system which attempts to incorporate power factor in a control system for controlling electrodes in electric arc furnaces is disclosed by U.S. Pat. No. 3,662,075 to Sakai et al. (hereinafter xe2x80x9cthe Sakai patentxe2x80x9d). The electrode control system disclosed by the Sakai patent includes an electrode driving mechanism, an automatic current regulator responsive to the current flowing through the electrode, and a program control unit connected between the automatic current regulator and the electrode driving mechanism for transmitting electrode control signals to the electrode driving mechanism. A power factor regulator responsive to the actual power factor of the furnace is connected to the program control unit to provide an error-correcting signal to the program control unit. The program control unit transmits electrode control signals to the electrode driving mechanism based on the current flowing through the electrode to raise or lower the electrode. The actual power factor of the electrode is transmitted by the power factor regulator to the program control unit to ensure that the electrode control signal sent to the electrode driving mechanism by the automatic current regulator moves the electrode in the correct direction, i.e., up or down, in the electric arc furnace. The correcting signal generated by the power factor regulator is provided because of the errors associated with regulating the electrode based on operating current. For example, under certain operating conditions, the automatic current regulator may provide an incorrect xe2x80x9clowerxe2x80x9d signal to the program control unit which is xe2x80x9ccorrectedxe2x80x9d by the correcting signal generated by the power factor regulator and sent to the program control unit. Hence, the program control unit disclosed by the Sakai patent requires two input signals, a primary xe2x80x9ccontrolxe2x80x9d signal from the automatic current regulator and a secondary or correcting signal from the power factor regulator, and, consequently, does not regulate purely on power factor. The control system disclosed by the Sakai patent is primarily a current based regulating system which utilizes power factor only secondarily to correct errors.
While systems are known in the prior art which attempt to incorporate power factor into a control system for controlling electrodes in electric arc furnaces, these prior art systems utilize power factor only as a secondary or correcting signal, or used to control the power source of the electric arc furnaces. In addition these prior art systems only generate a signal indicative of power factor when the actual power factor of the electrode falls outside of a power factor range. Consequently, because these prior art systems cannot establish and maintain a preselected power factor set point, optimum working/operating conditions of the electric arc furnaces are never met.
In view of the foregoing, it is an object of the present invention to provide an electrode control system and method for controlling the positioning of electrodes in electric arc furnaces based on direct power factor regulation. It is also an object of the present invention to provide an electrode control system capable of both impedance based regulation and direct power factor regulation.
The above objects are accomplished with an electrode control system for an electric arc furnace and a method of controlling the position of an electrode in an electric arc furnace in accordance with the present invention. In particular, the present invention relates to an automatic electrode regulation or control system for regulating electric arc, or xe2x80x9carc lengthxe2x80x9d, of electrodes in electric arc scrap metal furnaces and electric arc ladle furnaces of the alternating current (AC) type. The electrode control system of the present invention includes a current transformer for measuring operating current of the electrode and a voltage transformer for measuring operating voltage of the electrode. An active power transducer is connected to the current transformer and the voltage transformer for calculating active power of the electrode from the measured operating current and operating voltage as a first output signal. A reactive power transducer is connected to the current transformer and the voltage transformer for calculating the reactive power of the electrode from the measured operating current and operating voltage as a second output signal. A programmable control unit is connected to the active power transducer and the reactive power transducer for receiving the first and second output signals. The control unit is configured to calculate actual power factor of the electrode from the first and second output signals and compare the actual power factor with a preprogrammed power factor set point. The control unit is further configured to generate a control signal indicative of the difference between the actual power factor and the power factor set point. An electrode positioning mechanism is provided for controlling movement of the electrode. The electrode positioning mechanism is connected to the control unit for receiving the control signal. The electrode positioning mechanism is configured to adjust a vertical height of the electrode such that the actual power factor approximates the power factor set point.
The control unit may further include a power factor calculation unit connected to the active and reactive power transducers for receiving the first and second output signals. The power factor calculation unit may be configured to calculate the actual power factor of the electrode and generate a third output signal indicative of the actual power factor of the electrode. The control unit may further include a controller connected to the power factor calculation unit for receiving the third output signal. The controller may be configured to compare the third output signal with the power factor set point and generate the control signal for the electrode positioning mechanism.
The electrode positioning mechanism preferably controls the height of the electrode relative to a level of molten metal contained in the electric arc furnace. The electrode positioning mechanism may be an electromechanical device operatively connected to the electrode for adjusting the vertical height of the electrode. The furnace transformer may include a secondary side. The current transformer and voltage transformer may be connected to the furnace transformer at the secondary side for measuring the operating current and operating voltage of the electrode. The programmable control unit may be a programmable computer.
The present invention is also a method of controlling the position of an electrode in an electric arc furnace having a furnace transformer and an electrode positioning mechanism. The method may include the steps of: determining active power of the electrode from operating current and operating voltage of the electrode; determining reactive power of the electrode from the operating current and the operating voltage of the electrode; calculating the actual power factor of the electrode from the active power and the reactive power of the electrode; comparing the actual power factor with a preselected power factor set point for the electrode; providing a control signal indicative of a difference between the actual power factor and the preselected power factor set point to the electrode positioning mechanism; and controlling a vertical height of the electrode with respect to a level of molten metal in the electric are furnace with the electrode positioning mechanism based on the control signal such that the actual power factor of the electrode approximates the power factor set point.
In addition the present invention is an electrode control system capable of both impedance based regulation and power factor based regulation in an electric arc furnace. Accordingly, the electrode control system discussed previously may further include an electrode impedance calculation unit connected to the current transformer via a current transducer and the voltage transformer via a voltage transducer for calculating electrode impedance from the measured operating current and the operating voltage. The programmable control unit may be connected to the active power transducer, the reactive power transducer and, further, connected to the electrode impedance calculation unit. The control unit may be configured to calculate the actual power factor of the electrode from the active and reactive power of the electrode and compare the actual power factor with a preprogrammed power factor set point. The control unit may be configured to compare the electrode impedance with a preprogrammed impedance set point. The control unit may also be configured to generate a control signal indicative of a difference between the actual power factor and the power factor set point or the difference between the electrode impedance and the impedance set point. The electrode positioning mechanism may be connected to the control unit for receiving the control signal, and may be configured to continuously adjust the vertical height of the electrode such that the actual power factor approximates the power factor set point or the electrode impedance approximates the impedance set point.
Furthermore, the present invention is a method of controlling the position of an electrode in an electric arc furnace based on impedance or power factor regulation. Accordingly, the previously discussed method may further include the steps of determining electrode impedance from the operating current and the operating voltage of the electrode; providing to the electrode positioning mechanism a control signal indicative of a difference between the actual power factor and a preselected power factor set point or the difference between the electrode impedance and a preselected impedance set point; and controlling a vertical height of the electrode with respect to a level of molten metal in the electric arc furnace with the electrode positioning mechanism based on the control signal such that the actual power factor of the electrode approximates the power factor set point or the electrode impedance approximates the impedance set point.
Further details and advantages will become apparent from the following detailed description, in conjunction with the drawings.